Ultrastructural changes in the enamel crystals occurring when "remineralizing" solutions are applied to partially decalcified enamel will be investigated. Alterations in crystals, etch pattern, and rate of dissolution will be examined. The effect of removal of the organic matrix of enamel using low temperature ashing will be studied. Rat enamel has shown changes in the acid etching of the interprismatic enamel and a similar effect may occur in human enamel. Correlated microradiographic and transmission electron microscopic studies on precarious enamel will be continued. The role that shortening of the c-axis of the enamel crystal may play in the pattern of spread of caries will be investigated.